What should I do if I catch my children watching pornography?
I assume you were speaking of adolescents. Two things about adolescents makes it especially difficult for them to resist looking at pornography even if they know it is wrong and even if they wish to avoid it; they have poor impulse control. The brain’s ability to control impulses does not fully mature until after the age of 25. The other thing is that adolescents have very high levels of sex hormones which makes pornography especially attractive to them.
There are good reasons to limit their access to pornography as you already know. But if they manage to bypass you one day and get to look at some, it is best to forgive and forget it. No good can be achieved by punishment if they are well-behaved children otherwise. Desire and curiosity overcame them and punishment will not stop this desire or curiosity. Instead of making a big deal out of it, it is best to make sure they cannot do it again without humiliating them about it.
Our aim should not be completely control their lives so that watching of pornography may never happen. There will likely be occasions when it happens despite our best efforts, and making a big deal of it may only increase their curiosity and their desire for it. Rather, we should have sensible policies in the home (such as no unmonitored Internet browsing) that make it habitual pornography-watching impossible. Beyond that, there really is not much we can do. Most Muslim children grow up to be decent humans even if they succumbed to their desire to view pornography a few or many times in their youth. As long as the pornography watching is not habitual and as long as the parents do make sure they do not have so much freedom that they can easily watch it, then it is not a big problem to worry about.